The need for maintaining the validity of data which is moved from one device to another over a time division multiplexed network is well known. If data becomes corrupt during transit, the receiving device can produce an improper response. Data is transferred over the network in bits represented by "1's" or "0's". Each bit occupies one time slot of a data frame and can be addressed individually as a data unit, or bits can be arranged in byte or word sized data units wherein each byte or word sized data unit is addressed individually. A frame sized unit is made up of a number of bit, byte or word sized data units. Data corruption usually occurs in the form of noise in the network transmission line which changes the state of one or more of the bits in a data unit being transferred on the network. In more sophisticated network operations, data is transferred between the host device (CPU) and field devices (I/O devices) in complete frames. In this type of network communication, each field device connected to the network reads the complete frame and then the addressed field device responds to the host device with a complete frame of information. The check sum or CRC error detection method is generally used for these more sophisticated networks. Parity error checking is also a commonly used method of checking for data errors on a network. Parity checking is less complicated than CRC error checking and can be used with bit, byte, word or frame length data units. Parity can be either ODD or EVEN. Parity checking is accomplished by adding the number of data bits having a value of "1" in the data unit to be transmitted and then adding the parity bit or bits required to get the desired ODD or EVEN total for the transmitted data unit. The receiving host or field device receives the transmitted data unit and calculates the parity bit or bits required to give the desired ODD or EVEN total for the received data unit. If the parity bit or bits calculated by the receiving device matches the parity bit or bits sent by the sending device the data is good. If the calculated parity bit or bits does not match the parity bit or bits sent by the sending device there is a parity error in the received data. Therefore, due to the nature of its error checking process, parity error checking has only a 50% probability of detecting data errors.
Time slot bleedover is another form of data error which can occur when data is transmitted over a network. In this application time slot bleedover is defined as data from one time slot corrupting data from the next succeeding time slot due to the inability of the network line capacitance to charge or discharge before data in the next time slot is read from the network bus. Time slot bleedover error checking is very important in bit, byte or word oriented networks wherein each field device reads only the bit, byte or word sized data unit in the frame which is addressed to it and provides a bit, byte or word sized output in response to the information read from the network. Time slot bleedover errors can be detected by the more sophisticated and expensive check sum and CRC detection methods but parity error checking with its 50% probability of detecting data errors is much less likely to detect a time slot bleedover error.
In time division multiplexed networks, it is also possible for transmission line noise or time slot bleedover to corrupt the multiplex channel address which is transmitted as a binary number. Therefore, it is important to check for data errors in the multiplex channel address as well as in the data units of the frame being transmitted on a particular multiplexed channel address.
It is possible that a network may contain field devices which are not capable of data error checking or which may not require data error checking as well as field devices which are capable of, and do require data error checking. In some applications, it can be desirable to disable data error checking for some or all field devices such that the maximum number of field devices can be connected to the network. Therefore, some method for selectively enabling or disabling the data error checking method should be provided to both the host and the field devices such that both data error checking devices and non-data error checking devices can operate on the same network.